Communication devices such as User Equipments (UE) are enabled to communicate wirelessly in a radio communications system, sometimes also referred to as a radio communications network, a mobile communication system, a wireless communications network, a wireless communication system, a cellular radio system or a cellular system. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications network.
User equipments are also known as e.g. mobile terminals, wireless terminals and/or mobile stations, mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity.
The wireless communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a network node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. eNB, eNodeB, NodeB, B node, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several radio access and communication technologies. The base stations communicate over the radio interface operating on radio frequencies with the user equipments within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: GroupeSpécial Mobile).
In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the user equipment. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
According to 3GPP/GERAN, a user equipment has a multi-slot class, which determines the maximum transfer rate in the uplink and downlink direction. GERAN is an abbreviation for GSM EDGE Radio Access Network. EDGE is further an abbreviation for Enhanced Data rates for GSM Evolution.
The ever increasing demand for higher data rates poses challenges to operators in how to evolve their existing cellular communications networks to meet the demand. A number of deployment options exist such as (1) increase the density of an existing macro base station grid, i.e. an existing grid of macro base stations, (2) increase the co-operation between existing macro base stations to mitigate interference, or (3) to deploy smaller and/or lower power base stations within the macro base station grid in specific areas where high data rates are needed. Such smaller and/or lower power base stations are often referred to as Low Power Nodes (LPNs) or pico base stations.
The first option (1) is complicated by the difficulty and cost associated with securing new macro sites, especially in urban areas. The second option (2) is complicated by the difficulty in securing a low latency link e.g. a fast link, between base stations to enable co-operation. In contrast, the third option (3), referred to as a heterogeneous deployment, or a heterogeneous network (HetNet), is appealing since it is often easier and more cost efficient to deploy small LPNs than to deploy macro nodes such as macro base stations.
FIG. 1 illustrates an example HetNet comprising a number of macro cells 10a and a number of pico cells 11a. The macro cell 10a defines a radio coverage area of a macro base station 10, and the pico cell 11a defines a radio coverage area of a pico cell 11. A layer or area comprising smaller base stations, or low power nodes (LPNs) is referred to as a pico layer or a pico area. The pico base stations are often placed strategically in areas with a high density of users requesting high data rates. Such areas are often referred to as hotspots.
In LTE systems, cell selection is based on the received power of the reference symbols transmitted in the downlink, e.g. Reference Symbol Received Power (RSRP) and measured by the User Equipment (UE). Due to the inherently low transmit power of the pico base stations, the pico cells 11a typically have a smaller coverage area than the macro cells 10a. For example, with a 40 W macro base station 10 and a 1 W pico base station 11, as schematically illustrated in FIG. 2, the power imbalance between them is 16 dB which produces vastly different coverage areas. This creates challenges in the deployment of the low power nodes, since unless they are deployed to exactly cover the hotspots, they will not be able to serve so many users.
Another issue with the power imbalance, e.g. the power difference, is that while users may connect to the base station with the best downlink, they may not always connect to the base station with the best uplink. Such an uplink/downlink imbalance, i.e. an imbalance between the uplink and the downlink, is illustrated in FIG. 2. The dashed bold line 12 represents the cell border from a downlink perspective, based on measured RSRP values at a user equipment 13. In other words, the dashed bold line 12 schematically illustrates the cell edge when cell selection is based on downlink received signal strength. Notice that the downlink cell border is closer to the pico base station, pico eNB, 11 due to its lower transmit border. In contrast, from an uplink perspective, the cell border should occur at a position where the path loss to both the macro base station 10 and the pico base station 11 is the same. For the sake of illustration, this occurs roughly half way between the macro and pico base stations 10,11, as is illustrated by the dotted vertical line in FIG. 2. In other words, the dotted vertical line in FIG. 2 schematically illustrates the cell edge when cell selection is based on uplink path loss. The transition zone between these two borders is the area in which a UE 13 under normal circumstances would have the best downlink from the macro eNB 10, but not the best uplink. The best uplink is to the pico eNB 11 since the path loss is less to the pico eNB 11 than to the macro eNB 10.
In certain circumstances, which will be elaborated below, it may be beneficial to bias the cell selection towards the pico base station 11 when the UE 13 is in the transition zone. To enable such offloading of UEs from the macro base station 10 to the pico base station 11, a Cell Selection Offset (CSO) may be configured. For example, say the UE 13 measures an RSRP from the pico base station 11 to be x dB and to the macro base station 10 to be y dB, with x<y. Under normal circumstances, the UE 13 would connect to the macro base station 10. However, the UE 13 may be forced to connect to the pico base station 11 if the CSO is added to x to make the adjusted RSRP from the pico base station 11 greater than the RSRP from the macro base station 10. Such a process is referred to as range expansion, since the range of the pico cell 11a is increased. If the offset is less than or equal to the macro/pico power imbalance, i.e. the power imbalance between the macro cell 10a and the pico cell 11a, then the uplink is improved since the path loss to the pico base station 11 is less than the path loss to the macro base station 10.
The trade-off is that the downlink may be penalized. However, it may still be beneficial to allow range expansion in certain circumstances. One circumstance is when the macro cell 10a is heavily loaded and the pico cell 11a is nearly empty. In that case, the UE 13 may still obtain reasonable data rates in the downlink since there are more radio resources available from the pico base station 11. Another motivation might be when the pico base station 11 is not placed exactly in the middle of a hotspot, so the use of range expansion may better ensure uptake of traffic. Yet another reason is when resource partitioning is employed in which the macro base station, e.g. macro eNB, 10 is silent or almost silent in a certain set of subframes such that interference to the DL transmissions from the pico base station 11 is avoided. Such an approach has been introduced in LTE Rel-10, and is referred to as Almost Blank Subframes (ABS).
The above discussion has focused on LTE. However, range expansion may also be configured in HSPA networks, but there it is referred to as extended Soft HandOver (SHO). One of the main differences between LTE and HSPA is that macro diversity or SHO is employed in the uplink. In other words, the UL transmissions from a UE are received, detected, and decoded by two or more base stations. The collection of links participating in SHO is referred to the active set. The process of establishing a new SHO link is further referred to as an active set update.
The implication of SHO in a HetNet scenario, is that if a UE 13 is served by the macro base station 10 in the downlink, a SHO link may be established to the pico base station 11 in the uplink without necessarily changing the serving cell away from the macro base station 10. Moreover a similar offset to the CSO discussed above may be configured such that links to a pico base station 11 are added sooner than under normal circumstances to take advantage of the better path loss to the pico base station 11 on the SHO link. By establishing such a SHO link to the pico base station 11, the uplink is helped, but the downlink is largely un-penalized in contrast to LTE. Note that the downlink is not completely un-penalized. One issue is that certain downlink control channels associated with the SHO link to the pico base station 11, such as Fractional Dedicated Physical Channel (F-DPCH), E-DCH Relative Grant Channel (E-RGCH), and E-DCH Hybrid ARQ Indicator Channel (E-HICH), may still be affected by interference from the macro base station. This sets an upper limit on the degree of SHO extension possible. This issue is not further discussed herein.
FIG. 3 illustrates the uplink and downlink cell borders in a similar fashion as in FIG. 2, except here circles indicate the area in which a UE 13 is likely to be in soft handover between both the macro base station 10 and the pico base station 11. In FIG. 3, the macro base station 10 steers power control of the UE transmitting HS-DPCCH, and the HS-DPCCH is received at serving macro base station 10 with sufficient quality. Further, in FIG. 3, the SHO region is schematically illustrated as the region between the innermost circle and the circle that falls between the equal pathloss border and the equal received CPICH border, Thus, this occurs in a fairly narrow strep around the downlink cell border where the receive power of the common pilot, e.g. Common Pilot Channel (CPICH), from both the pico and macro NodeBs 10,11 is equal. The conventional SHO region is fairly narrow, and is defined by a set of thresholds that determine when SHO links are added to the active set.
FIG. 4 shows the case when an offset is added to the threshold to allow soft handover links to be established to the pico base station 11 sooner than in a conventional situation. In FIG. 4, the pico base station 11 steers power control of UE transmitting HS-DPCCH, and the HS-DPCCH is received at serving macro base station 10 with insufficient quality, thus requiring boosting. In this case, no offset is employed for adding SHO links to macro base stations 10. The effect is to extend the SHO region outwards from the pico base station 11. As discussed earlier, and emphasized here again, the UE 13 in the extended SHO region is still served by the macro base station 10 in the downlink since it is to the left of the DL cell border.